For each FPU instruction that can trigger an FPU exception, to call
call update_fcr31() after.

Remove the manual NaN assignment in case of float to float operation, as
softfloat is already taking care of that. However for float to int
operation, the value has to be changed to the MIPS one. In the cvtpw_ps
case, the two registers have to be handled separately to guarantee
a correct final value in both registers.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>